\documentclass[12pt]{article}

\title {Simple \LaTeX Document}
\date{}
\author{Mark Canning}

\begin{document}
\maketitle

\center{this is  text centered}
\section{First Section}
\subsection{Introduction}
In this paper we will
produce some simple
mathematical formulae.

\begin{equation} 
\label{eqn:i}
\sum_{i=1}^n i
\end{equation}
We would like to use the summation laws 
to simplify Eqn.~\ref{eqn:i}

\begin{enumerate}
\item This is item 1
\item This is item 2
\item This is item 3
\end{enumerate}

\begin{verbatim}
int main(int argc, char** argv)
{
	// Block/Local scope, modifiable 
	int r = 10;

	return 0;
}
\end{verbatim}

\begin{equation} 
\label{eqn:n}
\sum_{i=1}^n n
\end{equation}
This simplifies to
\begin{equation} 
\label{eqn:n2}
n^2
\end{equation}
\section{Second Section}
\end{document}
